In radiofrequency electrosurgical cutting a radiofrequency current is allowed to pass from an active cutting electrode through a patient's tissue and into a grounding pad or cable. The current cuts tissue at the active cutting electrode, the cutting rate being dependant on current density through the tissue in that area. With a low current density heat is generated but no cut is achieved. With a high current density fast cutting occurs.
Current density depends upon the voltage applied to the electrosurgical circuit and the series impedance or resistance to current flow of that circuit. Current density is also dependent upon the area the active cutting electrode presents to the patient's tissue. The smaller this area, the higher the current density. Since the area of the active electrode is fixed for a specific cutter, and the series impedance of the circuit is beyond the surgeon's control, the current density is typically adjusted by varying the voltage applied to the electrode. This adjustment is typically present on conventional electrosurgical generators.
The series impedance is dependent upon several factors which are outside the control of the surgeon. These factors may include the material and design of the active electrode, the type of tissue to be cut, and the location of the grounding pad relative to the cutting site. Generators used in this type of surgery have a wide range of power output to accommodate a variety of procedures and devices. For example, the generator can be adjusted to either cut tissue or to merely cauterize previously cut or torn tissue.
The objective in electrosurgical cutting is to heat the cells of the tissue so rapidly that they explode into steam leaving a cavity in the cell matrix. The heat is meant to be dissipated in the steam and not to dry out adjacent cells. When the electrode is moved and fresh tissue is contacted, new cells are exploded and the incision is made. The current utilized in electrosurgical cutting is in the radiofrequency range and operates by jumping across an air gap to the tissue. This is commonly referred to as sparking.
An explanation of electrosurgical cutting theory can be found in the FORCE 1 Instruction Manual published by Valleylab of Boulder, Colo., on Mar. 1, 1986.
An advantage of electrosurgical cutting, particularly if it is performed utilizing a cutting electrode as disclosed in application Ser. No. 07/522,254 filed May 11, 1990, now U.S. Pat. No. 5,080,660 is that overheating of adjacent tissue with accompanying desiccation and damage is limited or prevented. Thus, this procedure provides a clean cut without damage to adjacent tissue. A clean controlled cut is particularly desirable to assure that tearing does not occur in a direction away from the desired orientation of the cut.
Dilatation catheters are used to dilate body vessels, orifices and conduits such as an artery narrowed by atherosclerotic plaque or a urethra constricted by an enlarged prostate. These catheters basically consist of an elongate cannula having an inflatable non-extensible balloon or bladder at or near its distal end. A guide wire or other axial support means is often included to improve the torque control or "steerablilty" of the catheter.
The major advantage of using a dilatation catheter instead of conventional surgery is that it is less invasive. Nevertheless, the dilatation processes of the past can also result in significant trauma. As the elastomeric bladder expands, it exerts pressure on the surrounding tissue, causing the tissue to compress, deform and expand. The tissue, of course, has an inherent limit of deformability. When the dilation pressure causes the tissue to deform beyond that limit, the tissue tears apart, often to form a jagged wound. A principal object of the present invention is to provide a dilatation catheter that permits tissue to be stressed, even beyond its limit of deformability, without experiencing uncontrolled tearing and the undesirable conditions associated therewith.
U.S. Pat. No. 4,747,405 issued to Leckrone on May 31, 1988, U.S. Pat. No. 4,669,469, issued Jun. 2, 1987 to Gifford, III., et al., and PCT/US 86/02617 application of Leckrone, published Jun. 16, 1988, are each concerned with atherecotomy devices wherein a balloon is used to position an opening in a casing about an obstruction such as plaque. The balloon does not carry a cutting element to incise tissue but does carry means for disintegrating the plaque which is generally entrapped within a hole in the casing. The balloon basically positions the hole in the casing up against and about the plaque. Thus, the balloon is not symmetrically located within the blood vessel, an outward cutting element is not present, and the blood vessel is not torn by the dilation force.
U.S. Pat. No. 4,799,479, issued Jan. 24, 1989 to Spears, shows use of a balloon to open up an artery and then utilizes a laser, heated wire mesh, or the like, to heat up blood trapped between the media and the plaque so that dilation will be maintained and so that a smooth wall will result.
U.S. Pat. No. 4,273,128, issued Jun. 16, 1981 to Lary, teaches the use of a balloon with a knife blade, or a series of knife blades, longitudinally distally removed from the balloon.
Soviet Patent 599802 published in 1976 utilizes a balloon which is located within a tube. When the balloon is extended this forces a cutting element through a window in the tube to accomplish fenestration. Balloon pressure is not exerted against body tissue as the balloon is expanded within the tube.
German Patent 3,402,573 is concerned with a single lumen multi-purpose catheter having an extensible elastic balloon with a cutting facility for treatment of stenosis. This patent utilizes three balloons of equal size at the distal end of the catheter. Each elastomeric balloon carries small cutter elements which extend in the longitudinal direction and which are held in a trough made of hard rubber or plastic. Prior to use the cutters lie hidden in longitudinal slots of the relatively thick wall of the one-lumen catheter. Threads anchor the plate when the balloons are inflated thereby limiting the degree of penetration of adjacent plaque and possibly tissue.
U.S. Pat. No. 4,484,579, issued to Meno, et al. on Nov. 27, 1984, is concerned with a commissurotomy catheter which serves for separating fused heart valve leaflets. The device includes four balloons carried by a single catheter structure. In use the device fits through the valve with two balloons on each side of the valve. A nylon or similar string is attached between the pairs of balloons on each side of the valve. The balloons can be alternately expanded and contracted thereby causing the strings strung between each pair of balloons to saw or pulsate into fused portions of the heart valve leaflets and separate them. The actual cutting portion of the string is not carried on the exterior of the balloons.
The above-mentioned patents do not make use of an electrosurgical or radiofrequency cauterizing or cutting element. Nor do the above patents either suggest or show any advantages for utilizing an inextensible bladder or balloon, i.e., a balloon which is not elastomeric (or elastic) and which can be inflated to only a selected shape and volume. Furthermore, the above discussed patents are not concerned with a radially symmetrically, generally cylindrical in shape when expanded, balloon which extends longitudinally along a body passage and a cutting element which extends longitudinally along and generally parallel to the balloon, which balloon creates a substantially uniform tangential tension in tissue being cut, and which cutting element at the same time performs the necessary cutting whereby a clean longitudinally extending incision results and uncontrolled tearing of the tissue does not occur.
The present invention is directed to overcoming one or more of the problems as set forth above.